Heat transfer/cooling fluids are commonly used in industry, technical installations in buildings, refrigeration equipment and motor applications.
Good heat transfer properties are important in heat transfer/cooling fluids. This presupposes a good specific thermal capacity, thermal conductivity and pumpability. A low viscosity value at low temperatures is typical of heat transfer fluids, because in such a case the fluid can be brought into a turbulent flow with a lower pump power.
In addition to the above properties, important properties of heat transfer fluids often also include environment-friendliness and non-toxicity, in particular when used in the food industry and in the heating of household water. The heat transfer fluid must not be in any way harmful to the products being processed. Slight leakages of the fluid into the process could cause severe accidents. Therefore non-toxicity of the fluid is one of the most crucial factors.
The most typical fluids are organic and water-containing heat transfer/cooling fluids. The problem with organic fluids is the difficulty in their use and their toxicity. With respect to most of its properties, water is a nearly perfect heat exchange fluid. It has good heat transfer and pumpability properties. In addition, water is completely non-toxic and non-hazardous to the environment. In addition, it is in no was flammable. Indeed, the major disadvantage of water is the narrow temperature range for its use. As is known, water freezes at 0.degree. C. and boils at 100.degree. C. Furthermore, water causes corrosion, especially in connection with iron-containing materials.
As to the state of the art regarding heat transfer fluids used at low temperatures, reference can be made to, for example, EP application publication 0641849, which discloses a heat transfer fluid composition comprising an alkylmethylsiloxane fluid or a combination of alkylmethylsiloxane fluids and various fluids based on polydiorganosiloxane with terminal trimethylsilyl groups.
The most typical water-based heat transfer fluids are aqueous mixtures of ethylene glycol, propylene glycol and ethanol. Ethylene glycol is one of the best known heat transfer fluids, and it is used in particular in the automobile industry. However, the use of ethylene glycol has disadvantages in that the substance is toxic and is not very environment-friendly. Propylene glycol is indeed often used instead of ethylene glycol in applications in which less toxic properties are required. Although propylene glycol is relatively non-toxic, it is, nevertheless, a substance which pollutes the environment. One disadvantage of propylene glycol is the great increase in its viscosity at low temperatures; this increases the required pump power.
The non-toxicity of ethanol as compared with ethylene glycol is an advantage in its use, but its use has the disadvantage in its great volatility and consequently in its constituting a fire hazard and great increase in its viscosity at low temperatures, the latter being, however, more advantageous than with propylene glycols. For this reason ethanol is a commonly used substance as a heat transfer fluid in laboratories and in conditions requiring non-toxicity. However, the use of ethanol involves the problem that it requires permits from the health authorities, a factor which complicates the use of this fluid.
Corrosion, in particular in the case of glycols, has forced users to seek expensive and effective corrosion inhibitors. The monitoring of the composition and concentration of corrosion inhibitors is difficult. In general, an effective inhibitor makes an otherwise highly non-toxic liquid toxic. Typically complicated solutions increase the cost of the end solution.
As regards the state of the art associated with corrosion inhibitors, reference can be made to EP application publication 0369100, which discloses a heat transfer fluid which contains a dicarboxylic acid mixture as a corrosion inhibitor and in which the cooling fluid composition comprises a water-soluble liquid alcohol to lower the freezing point; for example ethylene glycol and a mixture of glycol and diethylene glycol have been used in that capacity.